1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an image transferring method and toner for an image forming apparatus.
2. Description of the Background Art
Generally, an electrophotographic image forming apparatus needs a number of image forming steps. A copier, for example, converts a document image to an electric signal with a scanner or optics while a printer directly inputs a document image in a plotter in the form of a signal. Writing means scans a photoconductive element or image carrier with a laser beam in accordance with the electric signal to thereby form a latent image. Toner or similar fine colored powder is deposited on the latent image to thereby produce a corresponding toner image. The toner image is then transferred from the photoconductive element to a sheet or recording medium. Today, it is a common practice to sequentially transfer toner images of three to four different colors to an intermediate image transfer body one above the other and then transfer the resulting composite color image to a sheet. In any case, the toner is fixed on the sheet by heat and pressure.
The consecutive steps stated above all involve the causes of image quality degradation. Particularly, development and image transfer noticeably deteriorate image quality, as known in the art. More specifically, during development, toner electrostatically deposits on the photoconductive element under the action of an electric field on the photoconductive element. Therefore, it is likely that the toner deposits on the photoconductive element over a larger area than the latent image or that the toner image is blurred due to rubbing of carrier grains. Recently, this problem is coped with by reducing toner grain size, using spherical toner or reducing carrier grain size by way of example.
As for image transfer, a sheet, conveyed in synchronism with the movement of the photoconductive element carrying the toner image thereon, is brought into contact with the photoconductive element, so that the toner image is electrostatically transferred to the sheet by an electric field. The problem with image transfer is that the toner is electrostatically scattered at positions preceding and following the position where the sheet and photoconductive element contact each other or that the toner image is blurred. While image quality is degraded during fixation as well, this degradation has customarily been improved by using an elastic fixing roller or reducing a nip for fixation by way of example.
Various schemes have heretofore been proposed to improve image quality in relation to image transfer. Japanese Patent Laid-Open Publication No. 2000-155472, for example, proposes a specific position of an image transfer roller and specific contact pressure. Japanese Patent Laid-Open Publication No. 2000-221800 proposes to press a float roller against a photoconductive element. Japanese Patent Laid-Open Publication No. 2001-209255 teaches specific volumetric resistance of an intermediate image transfer body and specific properties of toner. Further, Japanese Patent Laid-Open Publication No. 7-5776 discloses a method of applying an image transfer bias to a press roller while using an amorphous silicon photoconductive element and capsule toner. Moreover, Japanese Patent Laid-Open Publication Nos. 5-107796 and 6-230599 each propose a fixing method using capsule toner and a press roller.
However, the capsule toner and press roller scheme is not desirable because capsule toner has various problems in practical use. Particularly, development and fixation are not compatible with each other while cost is excessively high. While toner produced by polymerization has recently been proposed to uniform image quality, toner scattering, blur and other problems particular to image transfer and ascribable to discharge are left unsolved.